1. Field of the Invention
The present invention relates to a metal-made carrier body for carrying thereon an exhaust gas cleaning catalyst which is generally provided as an exhaust gas cleaning means at an intermediate point of an exhaust system of an automobile.
More particularly, the invention relates to a metal-made carrier body for carrying thereon an exhaust gas cleaning catalyst, which body is composed of constituent members which are inexpensive and are excellent in corrosion-resistivity and processability. The invention is also concerned with a process of producing such metal-made carrier body.
2. Description of the Related Art
Conventional metal-made carrier bodies of the above sort, which are adapted to carry an exhaust gas cleaning catalyst thereon, include those having a structure formed by laminating at least one sheet-like metal band and at least one corrugated metal band in layers (laminated type) or rolling them together (rolled type) into a multi-layered composite body with a view toward increasing the carrying area per unit volume, namely, with a view toward increasing as much as possible the effective area of contact between exhaust gas and the exhaust gas cleaning catalyst per unit volume and further reducing the weight of the metal-made carrier body to a maximum possible extent. As it is in the form of a honeycomb, this structure is hereinafter called "honeycomb core structure".
For example, a sheet-like metal band made of a heat-resistant thin steel sheet having a thickness of 0.1 mm or smaller and containing 20% of Cr and 5% of Al, and a corrugated band made from another thin steel sheet of the same type are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a honeycomb core structure defining many network-patterned gas flow passages along a central axis thereof for allowing exhaust gas to pass therethrough. The honeycomb core structure is enclosed within a tubular metal casing which has a single-layer structure and opens in opposite ends thereof. The members of the honeycomb core structure, i.e., the sheet-like band and corrugated band are put together into a vibration-proof structure. Namely, the sheet-like band and corrugated band as well as the honeycomb core structure thus rolled and the metal casing are put together at the areas of contact therebetween by welding, brazing or the like.
For the preferable thin steel sheet to be used as a constituent member of the honeycomb core structure, a heat-resistant stainless steel of Fe-Cr(20%)-Al(5%), and another heat-resistant stainless steel additionally containing Co or rare earth elements such as Ce, Y, etc. in order to improve oxadation proofness (U.S. Pat. No. 4,66,169 and U.S. Pat. No. 4,414,023) have been proposed. When using as a constituent member of the honeycomb core structure, a thin sheet of such known steel should have a thickness of about 0.05 mm (50 .mu.m) and a width of about 100 mm. If a stainless steel containing Al is used as a thin steel sheet, whiskers of Al.sub.2 O.sub.3 would be deposited on the surface of the thin steel sheet due to heat-treatment. The whiskers serve to assist in fixing a wash coat for carrying thereon an exhaust gas cleaning catalyst (Pt, Pd, Rh, etc).
Yet if an inexpensive heat-resistant steel such as a low-carbon steel, a low-chrome steel, a low-nickel steel, etc. is used as a thin steel sheet constituting the honeycomb core structure, an adequately satisfactory honeycomb core structure cannot be obtained because of its processability and corrosion-resistivity.
More specific problems are as follows:
i) If a heat-resistant steel containig Cr of 15 to 25% and Al of 2 to 5% is used, an adequately rigid corrugated band can be achieved, and especially there would be no deformation of the wave shape of the honeycomb core structure during the production of honeycomb core structure. Namely, when producing the laminated-type honeycomb core structure from a sheet-like steel band and a corrugated steel band, and also when inserting the honeycomb core structure in a tubular metal casing and fixing the honeycomb core structure on the tubular metal casing. However, this steel is very hard, and therefore a steel sheet is difficult to roll into a thin metal band for a honeycomb core structure. To obtain a steel band of a desired thickness, rolling and tempering must be repeated, which is laborious and time-consuming. Therefore it is expensive to manufacture a honeycomb core structure.
ii) In an attempt to obtain an inexpensive honeycomb core structure, it has been proposed to use a low-carbon steel containing less than 0.15% of carbon. A honeycomb core structure formed from the low-carbon steel is aluminized (forming solid solutions or alloying by dipping a honeycomb core structure in a molten aluminum liquid to cause mutual diffusion between the surface of the steel and the molten aluminum liquid). The resulting honeycomb core structure has no problem in corrosion-resistivity in an exhaust gas at a high temperature of 800.degree. to 900.degree. C. and also in cost of production. However, with this low-carbon steel, only a low-rigidity corrugated steel band can be obtained so that the wave shape of the corrugated steel band would be deformed markably when forming a honeycomb core structure.
Because of this markable deformation of the wave shape of the corrugated steel band, a desired height of the wave is difficult to achieve and therefore the mesh size of network-patterned gas flow passages in the honeycomb core structure would be reduced to cause disadvantages such as a pressure loss (lowering the efficiency of an internal combustion engine). Further, the contact between the sheet-like steel band and the corrugated steel band would change from spot contact to plane contact so that the amount to which an exhaust gas catalyst is to be carried on the carrier body can be reduced to impair the exhaust gas cleaning ability.
iii) Another attempt is to use an inexpensive heat-resistant steel such as a low-chromium steel, e.g. SUS410L (Cr content: 11 to 13.5%). This steel has a rigidity lower than the steel of i) above and higher than the steel of ii) above. Therefore, when forming a honeycomb core structure, the wave shape of the corrugated steel band would be deformed to a greater extent, compared with the steel of ii) above. The most significant problem of this steel is that it is inadequate in heat-resistivity and also in corrosion proofness. The same thing can be said when a low-nickel steel such as SUS201 and SUS202 are used as a heat-resistant steel.